This invention relates to techniques for measuring impedance in electrochemical cells. More particularly, the invention is directed to apparatuses and methods used for taking internal impedance measurements of electrochemical batteries and cells with improved sensitivity and noise/electromagnetic immunity as compared to currently existing methods.
Electrochemical batteries and cells have very low internal impedance. This is true in different types of cells, including those based on either lead acid or nickel cadmium chemistries for which impedances can be on the order of milliohms (m.OMEGA.). For this reason, an effective method for measuring impedance must be highly sensitive to small impedance values while being immune to noise and electromagnetic circuit interference. Prior methods of impedance measuring normally utilize one of five different types of electrical circuits: (1) bridge circuits; (2) voltage dividers; (3) 4-wire connections; (4) short circuits; and (5) time constant circuits. However, each of these methods is limited by the inherent characteristics of the particular circuit type used in performing the impedance measurement.
Bridge circuits are commonly used to sense impedance changes in batteries. Such a bridge circuit 20 is depicted in FIG. 1, which shows the basic configuration of a circuit of this type which is powered by an AC voltage source V.sub.i. These circuits generally include impedance elements 22 that are located along first and second current paths 24 and 26, the impedance elements 22 being located on either side of voltage divider points where the voltages V.sub.A and V.sub.B can be measured. For battery impedance measurements, one of the impedance elements 22 in the bridge represents the battery being measured. The output of the bridge, V.sub.o, is the potential difference between V.sub.A and V.sub.B. The voltages V.sub.A and V.sub.B are related to the input voltage, V.sub.i by the relation ##EQU1##
under the condition that V.sub.o is equal to zero (i.e. V.sub.A =V.sub.B), so that EQU (Z1)(Z4)=(Z2)(Z3).
For example, one way of using this circuit is to make one of the impedance elements 22 adjustable and adjust the value of the impedance until V.sub.o is equal to zero. The problem with this type of operation is that it requires continuous adjustment of the element for each frequency at which the measurement is made. This is because battery impedance is not constant over the frequency spectrum of interest.
An automated system for handling such a procedure is complex and difficult to implement. This circuit is typically used by picking nominal values of the three known impedance elements 22 to maximize the output voltage swing as the battery impedance changes through the sweep of frequencies and usable life. The sensitivity of the output is maximized when Z2=Z3 and Z1=Z4. This implies that one of the impedance elements 22 must have a value that is the complex conjugate of the battery impedance.
Another limitation of bridge circuits relates to the fact that since internal impedance is very low for most cell types, voltage drops across the battery will also be very low. Fixing the values of all but one impedance element 22 and allowing only this battery impedance element to change implies that either V.sub.A or V.sub.B will remain constant. The bridge 20 reduces to a voltage divider for changes in the battery impedance. The output voltage is inversely proportional to changes in the battery impedance. Thus, as the impedance of the battery increases, output voltage becomes smaller. To get sufficiently large voltage drops at the output, a large amount of current is required. For example, if the magnitude of the battery impedance were 5 m.OMEGA., a 1 A current would be required to produce a 5 mV drop at the output.
Such a condition would place a high gain requirement on any sensing amplification equipment connected at the output of the bridge circuit. For example, the input impedance of such an amplifier would be the impedance of the bridge circuit 20 and would be very low due to the low battery impedance. Where such low input impedances are involved, such as those below 1.OMEGA., amplifiers become highly susceptible to electrical field noise, whether self-generated or from other sources. This condition is compounded where the input signal is also very low. Adverse interference effects can be expected regardless of whether BJT or FET input stages are used. Although the addition of a transformer across V.sub.o is typically recommended in cases of low input impedance, the addition of such a device tends to contribute to circuit impedance, lowering the circuit's sensitivity. Alternatively, where a sufficiently high turns ratio is present, an added transformer can reduce the bandwidth of the output signal produced.
Since bridge circuits do not easily permit impedance sensing without adjustment of the known impedance elements 22, the circuit has no more sensitivity than the voltage divider circuit. Thus, such circuits are normally only usable in laboratory settings where the impedance elements can be adjusted.
A second commonly used technique for impedance measuring uses a voltage divider circuit, which is typically preferred over bridge circuits when adjustment of impedance is not required. A voltage divider circuit 28 used for battery impedance measurements is shown in FIG. 2. The circuit 28, like most designs of this type, is driven by an AC current source 30 since voltage levels are typically in the range of millivolts and current in the range of amperes and thus amperage is easier to regulate than voltage. The circuit includes a sensing impedance Z.sub.s and a battery impedance Z.sub.b in a series loop 29 with the AC current source. Each sensing and battery impedance has a respective sensor 31 that connects to the series loop 29 at the respective impedance's point of positive and negative potential. Each sensor 31 is separated from the series loop 29 by capacitors 32 used to block the battery's DC signal. This technique involves two measurements: (1) measurement of the voltage V.sub.s across a sensing impedance Z.sub.s, permitting measurement of the loop current given the known size of Z.sub.s ; and (2) measurement of the voltage V.sub.b across the battery 34 being measured.
Voltage divider circuits used to measure battery impedance are limited by the same disadvantages as bridge circuits. Like bridge circuits, voltage measurements are taken in the millivolt signal level since batteries have very low impedance. Thus, voltage divider circuits, like bridge circuits, are susceptible to electrical field noise and have limited sensitivity.
A third technique utilizes a circuit known as a 4-wire or "Kelvin" connection. This is among the most frequently used techniques for measuring battery impedance and has been described in numerous patents and other references. The general configuration of a 4-wire connection 36 is shown in FIG. 3. In principle, this circuit is very similar to a voltage divider circuit, being driven by a current source 37. But the 4-wire connection 36 does not have a sensing impedance Z.sub.s. A battery 38 is interrogated with a current signal, and the voltage drop V.sub.b across the battery 38 is measured with a sensor 31 separated from the battery nodes by capacitors 32. As indicated above, for most lead acid and nickel cadmium cells, the internal impedance Z.sub.b is very low. This means that the battery 38 will be driven with amperes of current, and output signals will be on the order of millivolts of potential.
Most of the problems associated with bridge circuits and voltage dividers also apply to 4-wire connections. In fact, U.S. Pat. No. 5,821,757 to Alvarez et al. specifically addresses the problem of reducing electromagnetic interference (EMI) that adversely affects the 4-wire connection described in U.S. Pat. No. 5,281,920 to Warst with the addition of twisted, shielded-paired wires. Other attempts to reduce system noise have included the incorporation of ground isolation, the selection of driving frequencies away from known sources of electric field noise, and the combined techniques of windowing and averaging.
The fact that there is a need for each of these attempted remedies demonstrates the inherent limitations of this type of circuit. In such a system, larger output signals require a larger input current signal. For example, output signals on the order of tens of millivolts require input signals on the order of tens of amperes. However, sensitivity tends to be inversely related to the impedance of a battery. Since larger cell sizes ultimately lead to progressively smaller internal impedances, then for progressively larger cells, output voltages produced using the 4-wire technique tend to be smaller for the same input current. It follows that this technique is generally inadequate for using in a broad range of cell sizes.
Another technique used to measure battery impedance is the short circuit configuration. This configuration is less common than others described above and has been used in applications where internal impedance magnitudes have been on the order of hundreds to thousands of ohms, such as in lithium iodine batteries used in pacemakers and related devices. A simplified illustration of a short circuit 40 is shown in FIG. 4. The circuit has a switch 41 connected to the positive node of a battery 42 having an impedance Z.sub.b. The battery's negative node is grounded, while the switch 41 connects the positive node to a current mirror 43 and to ground 45. This technique simply involves taking a voltage measurement on the unloaded battery 42 followed by a measurement of the short-circuited current to calculate a measure of the battery's internal impedance Z.sub.b. The short circuit is only applied long enough to get an accurate enough measurement of the discharge current.
Although this technique is useful for calculating impedance in small, lithium iodine batteries, other larger battery types, including larger lithium iodine and most lead acid batteries, pose a serious explosion hazard when similarly short circuited. This technique is also limited in that it can only be used to get a bulk number to represent the battery's internal impedance, which eliminates all phase and frequency related information.
One additional technique that is commonly used to measure battery impedance is the time-constant method. As demonstrated in the example circuit in FIG. 5, this method is based on the concept of an RC time response of a battery 44 where R is contributed from a battery 46 and a capacitor 45 is a selected known value C. The battery is connected between ground and a normally-open switch 47 which is connected through capacitor 45 to ground. The charge V.sub.c across the capacitor 45 can be monitored through operational amplifier 49. In operation, switch 47 is closed, causing the battery voltage V.sub.b to discharge through battery resistance R.sub.b to charge capacitor 45. The time it takes to charge the capacitor 45 to the voltage V.sub.b is used to determine battery resistance R.sub.b since the capacitance C of capacitor 45 is known and the time .tau.=R.sub.b C.
This method has been incorporated into lithium iodine cells used in medical devices such as pacemakers. It includes switching a battery into a circuit with a parallel capacitor and then measuring the time response to determine the time constant, .tau.=RC.
As with other techniques, the battery's internal impedance is assumed to be a resistive element and the resulting measurement is reduced to a bulk number. No information about phase or frequency contributions is measured or determined. The technique is also limited in that there is a necessary tradeoff between capacitor size and processing speed of the detection circuit. A larger capacitor requires a larger amount of energy to be drawn from the battery, while the smaller the capacitors, the less time there is for the detection circuit to determine the time constant, affecting the sensitivity of the circuit. This relative dependence on the capacitor's size ultimately affects the circuit's sensitivity.
Most prior art methods of measuring internal impedance in batteries rely heavily on taking voltage measurements. Due to the very low impedance magnitudes involved, output signals are normally expected in the range of millivolts. This means that in order for most prior art methods to be operable, high gain amplifiers having a combination of low voltage signals and low input impedances to the amplifier must be used, implying a high level of susceptibility to noise and EMI. The related apparatus sensitivities of most prior art methods are also related to the impedance of the measured battery. As battery cells become progressively larger, internal impedance becomes smaller. Voltage measurements in turn become progressively smaller, thereby reducing the sensitivity of the measuring circuit. Although increasing input current can improve the output signal, such a step can be prohibitive since a magnification from amperes to tens of amperes may be required to achieve the desired effect.
Frequent measurements at high current levels not only impose a higher power requirement on the circuit, but also subject the battery to higher levels of energy. Such conditions can potentially contribute to heating and eventual disruptions in normal cell reactions. As confirmed by the number of past efforts to improve existing impedance measurement techniques, a new technique for measuring impedance is needed that is less sensitive to noise and EMI effects. Such a technique should also be less dependent on direct voltage measurements that are taken across the subject battery, while remaining usable for a variety of battery sizes and configurations.